What is a social letter of introduction?

a.
a cover letter accompanying a résumé that you give to a friend employed at a place where you’d like to work

b.
a short letter written by a friend to someone well known to your friend in a place you plan to visit

c.
a letter that you can send out over a social network in order to get more friends

d.
a lengthy letter written by a relative that describes to others the reasons why they might want to be friends with you